It has long been recognized that an exceedingly efficient technique for heating liquids is to directly inject steam within the liquid to be heated. When steam is injected directly into a liquid, one can realize almost one hundred percent of the BTU's in the steam which are absorbed directly into the liquid. Unlike indirect heating by means of, for example, a heat exchanger, there is no condensate retaining unused sensible heat. Because of this high heat-transferability, direct steam injection can save a great deal in energy costs.
Direct steam injection systems offer other benefits as well when compared to heat exchangers and comparable indirect heating systems. A direct steam injection system can provide very accurate temperature control within several degrees fahrenheit and are efficient in that scale buildup does not become an issue. Systems of this nature also tend to be more compact than comparable heat exchange devices.
There are four basic types of direct steam injection systems, namely, the sparger, the mixing tee, the Venturi and the modulating injection system. The sparger is the simplest system in that it generally consists of nothing more than a perforated pipe discharging steam in a vented storage tank. However, these systems are not without their disadvantages. For example, they must be operated at a set and constant flow rate to prevent the hammering effect observed in steam/water systems. This is the result of operating at steam and water pressures which are at or near equilibrium.
Mixing tees comprise nothing more than steam and water lines which join a common conduit. Because separate lines are used for each fluid, capital equipment tends to be expensive and inconvenient to install.
Venturi systems are generally more acceptable than those previously discussed, but should be operated under conditions of constant steam pressure, inlet water pressure and outflow demand. If they do not, a hammering effect can again be observed as the steam and inlet water pressures approach an equilibrium condition. In addition, changes in these variables can result in varying outlet temperatures which may not be desired.
Prior attempts have even been made to employ static mixers for direct steam injection into a liquid stream. However, as in the other prior art approaches, the results have proven spotty with instability and lack of control problems being manifest.
FIG. 1, labeled "prior art" was reproduced from an article appearing in Chemical Engineering in its June 28, 1982, issue. The article, entitled "Considered Direct Steam Injection for Heating Liquids" by Pick illustrates a variable-orifice injector system with modulating steam control. More specifically, cylindrical tube 10 is shown as possessing steam inlet 1 which injects the steam into an injector tube 2 and piston 5. The piston is biased by spring 3 which acts to block or free holes contained in injector tube 2 depending upon steam pressure emanating from inlet 1. Bias is maintained by spring 3, the overall effect being to heat water entering from cold water inlet 6 as it emanates from cylindrical tube 10 at outlet 4. In light of the fact that this product has tiny holes, depending upon the quality of the water or liquid being used, the system can become plugged.
It is thus an object of the present invention to provide a modulating injection system for the introduction of steam to a fluid without the drawbacks as experienced in prior devices such as those shown in FIG. 1.